<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
  <style>
    div{
      width: 100px;height: 100px;
      border: 1px solid red;
    }
    #d1{
      /*设置为绝对定位，脱离文档流，后面的元素会顶上来*/
      position: absolute;
      /*相对于窗口做偏移（默认）*/
      bottom: 0;
      right: 0;
    }
    #d2{
        width: 200px;height: 200px;
        background-color: red;
        margin: 100px 0 0 100px;
        position: relative;/*作参照物*/
    }
    #d2>div{
        width: 100px;height: 100px;
        background-color: blue;
        margin: 50px 0 0 50px;

    }
    #d2>div>div{
        width: 50px;height: 50px;
        background-color: green;
        position: absolute;
        top: 0;
        left: 0;
    }
  </style>
</head>
<body>
<div id="d2">
    <div><div></div></div>
</div>
<div id="d1">div1</div>
<div>div2</div>
<div>div3</div>

</body>
</html>